An image sensing device includes a pixel array including a plurality of unit pixel blocks each including a plurality of unit image sensing pixels arranged in the pixel array and structured to convert light into photocharges. Each of the unit pixel blocks includes a first sub-pixel block including a first floating diffusion region structured to hold the photocharges and a plurality of unit image sensing pixels sharing the first floating diffusion region, and a conversion gain capacitor arranged adjacent to one side of the first sub-pixel block. The conversion gain capacitor includes an impurity region coupled to an input node that receives a conversion gain signal, and a gate structured to surround the impurity region and coupled to the first floating diffusion region to change a gain of the first floating diffusion region in response to a change in the conversion gain signal.